This invention relates to corneal contact lenses and more particularly to multifocal corneal contact lenses.
Multifocal contact lenses are designed to correct a condition of advancing age known as "presbyopia." In a presbyopic eye, ability to focus at near distances, such as the normal reading distance, and in some cases at great distances is diminished. The loss of focusing capability is due to hardening of the eye's natural crystalline lens material.
Generally, multifocal contact lenses (usually either bifocal, trifocal or aspheric) are concentric or segmented in configuration. In a conventional bifocal contact lens of the concentric type, a first, centrally located, circular correction zone constitutes either distant or near vision correction, while a second annular correction zone surrounding the first zone provides the corresponding near or distance vision correction, respectively. In a conventional bifocal contact lens of the segmented type, the lens is divided into two somewhat D-shaped zones. Usually the upper area is for distant vision correction, whereas the lower area is for near vision correction. Such conventional segmented contact lenses require some sort of shifting of the lens relative to the eye to achieve acceptable visual acuity for both distant and near vision.
A trifocal contact lens has a third correction zone whose focal length is between the focal lengths of the distant vision and the near vision zones. The third correction zone may be termed a middle, or intermediate, distance correction zone.
Because of the multiplicity of foci, conventional multifocal contact lenses often produce a perceived image which is blurred. Such a situation worsens as illumination decreases, such as during night driving. With reduced illumination, the pupil enlarges in diameter, and consequently more light simultaneously enters the eye through both the distant vision correction zone and the near vision correction zone. This overlapping imagery increases the blur within the eye and reduces acuity and contrast of vision. The user's brain is usually not capable of ignoring the blurry portion of the image in favor of the focused portion.